A diesel engine performs so-called EGR (exhaust gas recirculation) to return an exhaust gas to the air-intake side to reduce the amount of NOx (nitrogen oxides) contained in the exhaust gas. To recirculate the exhaust gas to the air-intake side, the intake air amount of the diesel engine is required to determine the EGR rate. To measure the intake air amount of the diesel engine, a MAF (mass air flow) sensor is used (for example, refer to Patent Literature 1).